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Scorching hot Mercury looks icier than previously thought

With blistering daytime temperatures of up to 800° F (427° C), Mercury isn’t the first planet in the solar system you’d think to look for ice. But its slow spin, thin atmosphere and perpetually dark poles mean that on the night side, the mercury (pun intended) can plummet to -290° F (-180° C), creating the perfect conditions for icy deposits. Now a new study from Brown University has found that Mercury may be even icier than previously thought.

Ice on Mercury was first suggested in the ’90s, as an explanation for extremely reflective regions picked up by radar telescopes. These shiny spots were detected at the floors of several craters near the planet’s poles, where no direct sunlight ever fell, and the freezing temperatures in those dark patches was plenty cold enough for ice to form.

The Brown study analysed data gathered by the laser altimeter onboard NASA’s MESSENGER probe, which orbited Mercury between 2011 and 2015. That instrument is normally used to measure the altitude of terrain, but the team found that it could be calibrated to help detect ice in signals bounced off the surface at an oblique angle. These “off-nadir” readings suggest that three craters near Mercury’s north pole seem to be home to large, previously-unknown ice deposits, and surface reflectivity data from the area surrounding the craters seems to indicate many more smaller deposits scattered around.

“We suggest that this enhanced reflectance signature is driven by small-scale patches of ice that are spread throughout this terrain,” says Ariel Deutsch, lead author of the study. “Most of these patches are too small to resolve individually with the altimeter instrument, but collectively they contribute to the overall enhanced reflectance. The assumption has been that surface ice on Mercury exists predominantly in large craters, but we show evidence for these smaller-scale deposits as well. Adding these small-scale deposits to the large deposits within craters adds significantly to the surface ice inventory on Mercury.”

The researchers estimate that as much as 1,313 sq mi (3,400 sq km) of the three large craters could be covered in ice. They also isolated four smaller patches, each about 3 miles (5 km) across, which implies that there are many others across the surface of the planet, ranging from several kilometers to a few centimeters.

Just how all that ice got there in the first place is still a mystery, the team says. Like water and ice deposits on other bodies like the Moon, it may be the product of solar wind sprinkling the base molecules on the surface, which then combine to form ice. Or it may have hitched a ride on watery comets or asteroids that collided with the planet.

“One of the major things we want to understand is how water and other volatiles are distributed through the inner solar system — including Earth, the Moon and our planetary neighbors,” says Jim Head, co-author of the study. “This study opens our eyes to new places to look for evidence of water, and suggests there’s a whole lot more of it on Mercury than we thought.”